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A common ATX form factor personal computer system is modeled in detail. The flow and temperature fields inside the chassis are numerically investigated as a conjugate heat transfer problem. The computational effort is concentrated on the forced air cooling of the CPU using a heat sink. Three different commercial heat sink designs are analyzed by using commercial computational fluid dynamics software packages Icepak and Fluent. The grid independent, well converged, and well posed simulations are performed, and the results are compared with the experimental data. It is observed that flow obstructions in the chassis and the resulting air recirculation affect the heat sink temperature distribution. The specific thermal resistance values for the heat sinks are compared. It is observed that although they have different geometries, all of the three heat sinks have similar specific thermal resistances. The best heat sink is selected and modified in order to have a lower maximum temperature distribution in the heat sink by changing the geometry and the material. Especially, replacing aluminum with copper as the heat sink material improved the performance. The importance of modeling the entire chassis is demonstrated by comparing the simulation results with the results from a model of only the CPU-heat-sink-fan assembly.